The present invention relates to a torque split control system for a four-wheel drive motor vehicle having a central differential, and more particularly to a control system which provides two or more standard torque split ratios.
In a four-wheel drive motor vehicle, a torque split ratio between front and rear wheels is determined to coincide with a dynamic weight ratio of the front and rear wheels, so that power of the engine is effectively used at acceleration.
In a four-wheel drive motor vehicle based on a front-wheel drive with a front-mounted engine which has a static weight ratio of 60 (front):40 (rear), the ratio of the front torque T.sub.F to the front wheels and the rear torque T.sub.R to the rear wheels is determined in the ratio of 50:50 which is the same as a dynamic weight ratio thereof. In the four-wheel drive motor vehicle based on a rear-wheel drive with the front-mounted engine which has the static weight ratio of 50:50, the ratio of the front torque T.sub.F and the rear torque T.sub.R is determined 40:60 in the same ratio as the dynamic weight ratio. Accordingly, a central differential comprising bevel gears is employed in the former vehicle in which an output torque of the engine is equally distributed, and the central differential having a simple planetary gear device is employed in the latter vehicle.
The former vehicle ensures safe driving on a slippery road. If a differential lock device is provided for locking the central differential, the driving force of the vehicle is more improved. However, the steering ability of the vehicle is not particularly improved. That is, when the vehicle makes a turn at high speed under a differential lock condition, all four wheels may slip (slipping spin) at the same time, causing difficulty in driving.
In order to ensure driving stability of the vehicle, the torque to the rear wheels is set to a larger value than that to the front wheels by arranging the central differential with the simple planetary gear device, so that the rear wheels may slip first. Thus, the vehicle can be safely driven by the front wheels at a small torque while a power transmission to the rear wheels occurs, causing idling thereof.
Japanese Patent Application Laid-Open 63-176728 discloses a four-wheel drive motor vehicle in which a central differential comprising a simple planetary gear device is provided. An output of a transmission is transmitted to a carrier of the simple planetary gear device. The torque is split to the front wheels through either a sun gear or a ring gear and to the rear wheels through the other gear. The torque to the front and rear wheels is unequally split at the ratio determined by the difference between pitch circles of the sun gear and the ring gear. A fluid operated multiple-disk friction clutch as a lock device is provided for controlling the differential operation. A standard torque split ratio determined by the ratio of the pitch circles can not be changed, unless the diameters of the sun gear and the ring gear changes.
In order to increase the standard torque split ratio, the diameter of the sun gear is decreased or that of the ring gear is increased. However, in a power transmitting system where an axle differential for front or rear axles and the central differential are coaxially disposed, a plurality of shafts such as axles, a front drive shaft, an input shaft connected to the transmission and a rear drive shaft are disposed so as to penetrate the sun gear. Therefore, the diameter of the sun gear cannot be decreased. On the other hand, the ring gear cannot be large because of the limitation of space and of the required gear ratio. Consequently, the power transmitting system cannot be applied to various vehicles having different static weight ratio, such as a four wheel drive motor vehicle based on a front-wheel drive vehicle with a front-mounted engine, and the four wheel drive motor vehicle based on a rear-wheel drive vehicle with a rear-mounted and mid-shipped engine.
Moreover, since the split of torque to the rear wheels can not be set to a large value, the control range of the multiple-disk friction clutch becomes small. Consequently, a motor vehicle having good driveability and steerability can not be provided.
To the contrary, it is desirable for the standard torque split ratio to be able to be freely determined without dimensional restrictions so as to transmit a sufficient torque to the rear wheels. The driveability of the vehicle such as stability, cornering performance and steering feeling changes in dependency on whether the vehicle is driven in an understeering condition or in an oversteering condition. However, when only one standard torque split ratio is provided, only one of the conditions can be set, so that it is difficult to comply with various road conditions and other driving conditions. Thus, in order to drive the vehicle in a manner appropriate to the driving conditions, it is preferable to provide a plurality of the standard torque split ratios.